1. Field of the Invention
The present invention relates to a hydraulic feed-rate control apparatus, and in particular, to the apparatus for changing a feed rate from a creeping speed to a fast speed, and thereafter changing the fast speed again to the creeping speed.
2. Description of the Related Art
With respect to a hydraulic feed-rate control apparatus for rotating and reciprocating a rotary tool such as a drill, a tap, a reamer, and a mill, and controlling a feed rate of a drill unit and the like for machining a workpiece, there exists a control device capable of a two-stages adjustment of a fast speed and a creeping speed. The hydraulic feed-rate control apparatus feeds the rotary tool at the fast speed in drilling the workpiece by the rotary tool and changes the fast speed to the creeping speed just before the tool penetrates the workpiece (for example, Japanese Patent Laid-Open Publication No. 2011-666 and Japanese Utility Model Laid-Open Publication No. S61-7860).
However, a conventional apparatus feeds a drill at a fast speed, when the drill drills a workpiece, and shortens a drilling time thereof; the apparatus changes the fast speed to a slow speed just before the drill penetrates the workpiece; and the apparatus feeds the drill at the fast speed same as in drilling the workpiece even when the drill contacts the workpiece and starts drilling it. Therefore, there is a problem that a burr and a drilled-hole displacement tend to occur when the drill bites the workpiece at a start of drilling it.
Furthermore, with respect to a flow rate adjustment, since a minute flow-rate adjustment is required, there is a problem that: in a feed-rate control apparatus utilizing in particular a hydraulic pressure, a hydraulic oil made to be gel results in being clogged at a valve and a groove and affects a minute speed control when the oil is used for a long time.